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    Re: Question on Lunars
    From: George Huxtable
    Date: 2004 Oct 27, 14:52 +0100

    In describibg Halley's method for lunars, I referred to the
    >> [... ]line though the horns continues sweeping through the
    >> stars at constant speed.
    and Herbert Prinz asked-
    >This line is not necessarily perpendicular to the ecliptic. Does Halley correct
    >for this or confine himself to cases where it is?
    The Moon can indeed be 5deg or so off the ecliptic, and as long as the Sun
    is well-spaced, that line through the horns can deviate from being
    perpendicular to the ecliptic, by up to 5 deg, perhaps a bit more. That
    would have given rise to an error in the difference between ecliptic
    ecliptic longitudes of Moon and star, which would be greater as the
    passing-distance increased. Whether Halley had any way to correct for this,
    I just don't know (but rather doubt it). He has left us little information
    about the details of how he worked.
    >> Even parallax matters little
    >> if a star can be chosen which passes the horn-line near the time of Moon's
    >> meridian passage.
    >If the Moon passes through the meridian you can still have considerable
    >parallax in longitude. The question is whether the ecliptic is perpendicular to
    >the vertical through the Moon. If this is the case alone, the parallax in
    >altitude affects only the Moon's latitude and does not interfere with the
    >observation of the conjunction.
    Herbert is right. But I doubt if Halley was expecting to get his Moon
    ecliptic-longitudes to within a minute. It's hard to tell how accurately
    his longitudes turned out. They enabled him to plot a point in mid-ocean
    where he could measure variation. Bat as he wasn't carrying a GPS receiver,
    it's hard for us (or him) to know how accurate his positions were.
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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